The invention relates to a circuitry for processing, in a television signal, a first, low-frequency synchronizing signal on which a second, high-frequency synchronizing signal is superposed.
A customary baseband television signal contains a synchronizing signal for synchronizing the line or frame frequency in addition to the luminance and the chrominance signal. This synchronizing signal consists of individual or grouped, basically square-wave pulses which occur at a low repetition rate compared with the overall television signal. For certain applications a second, high-frequency synchronizing signal, which preferably consists of sections of a sinusoidal oscillation having a frequency equal to an integral multiple of the line frequency of the television signal is superposed on the pulses of this synchronizing signal. This second synchronizing signal which is alternatively denoted as "pilot burst" has a constant phase throughout the overall duration of the television signal. Beginning and end points of the sections of the sinusoidal oscillation are fixed within the duration of the pulse of the first synchronizing signal, the amplitude of the sinusoidal oscillations basically corresponds to the amplitude of the pulses of the first synchronizing signal, and the zero line of the sinusoidal oscillations is located at the maximum, that is to say at the amplitude value of the pulses of the first synchronizing signal.
When such a television signal is processed by means of receiver arrangements suitable for, for example, the PAL or the SECAM standard, a problem is that the second synchronizing signal is detected because of its high amplitude as an interference of the first synchronizing signal, or that instead of a single pulse of the first synchronizing signal a plurality of these pulses are erroneously detected. In these cases the processing of the television signal is disturbed.
The JP-Kokai No. 58-119287 discloses a circuit for generating a command signal for eliminating the "pilot burst", in which selectively a frequency component corresponding to a synchronization pulse peak level in a PAL-color television HF signal is included. To that end, using a selector switch, either the received HF signal or the HF signal from a preceding line of the television picture is selected and applied to a frequency demodulator as well as to a circuit arrangement for generating a control signal for the elimination of the "pilot burst". The intermediate frequency of a narrow band filter included in the circuit arrangement for generating the command signal corresponds to the frequency in correspondance with the synchronizing pulse peak level of the video signal obtained by frequency demodulation of the HF signal. On reception of a frequency corresponding to the synchronizing pulse peak level, a signal of a high amplitude is consequently obtained which is detected by a signal level detection circuit and generates therefrom a command signal for cancelling the "pilot burst". In a cancelling circuit the "pilot burst" in the synchronizing signal of the video signal is removed by the command signal.